Bile salt micellar solutions play an important role in the solubilization of lipids. The bile salt lecithin micellar solution is of particular interest, because it solubilizes cholesterol, an otherwise insoluble substance. Since the solubility of cholesterol is an important determining factor in the development of hyperlipidemia, factors which control cholesterol solubility may affect the progress of atherosclerosis and coronary heart disease. Gallstone disease frequently occurs when the bile is not capable of sustaining cholesterol in solution in the gallbladder. New information on these bile acid solutions may therefore, have important implications in the treatment or prevention of disease. It is proposed, therefore, to further characterize these solutions by studying their thermodynamic properties. Calorimetric measurements are proposed to determine apparent molal heat contents of aqueous solutions of the taurine and glycine conjugated cholic, deoxycholic, and chenodeoxycholic acids and their sodium salts. These are the main bile acid constituents of human bile. Osmotic and activity coefficients of these solutes will be determined using the gravimetric isopiestic vapor pressure equilibration technique. The ternary systems bile acid-lecithin-water will be studied by the same techniques to obtain thermodynamic data for the bile acid-lecithin micellar solutions. Densities of these solutions will be measured in order to obtain partial molar volumes as a function of composition of these systems. The solubility of cholesterol in these micellar solutions will be studied, and thermodynamic quantities for the solubilization of cholesterol, other lipids, and some hydrophobic drugs will be estimated.